This invention relates to airflow cooling systems for electrical and electronic equipment and to cooling systems for fault-tolerant apparatus.
Modern electronic product packages typically have electronic components mounted on a set of printed circuit boards or other carriers that are arranged parallel to each other. In order to achieve design goals, the component density on the carriers has steadily increased. In addition, for purposes of electromagnetic shielding and mechanical isolation, the components are usually enclosed in a conductive metal box. Because the components are enclosed, a cooling system must be used to keep the components within an acceptable operating temperature range. Conventional cooling systems use multiple fans to generate a sufficiently high airflow across the components to insure that they do not overheat.
However, the high heat load generated by the high-density components combined with a demand for high system reliability and availability place severe demands on this cooling system. In particular, it is not currently acceptable to require an entire system to shut down if a single cooling fan fails because the system will be unavailable during the time it takes to replace the fan. Consequently, the prior art has developed cooling systems that attempt to continue operation of the system without overheating the components even after a fan has failed.
For example, one cooling system uses trays of axial fans located at opposite ends of the enclosed space. The fans are often used in a xe2x80x9cpush-pullxe2x80x9d arrangement in which the xe2x80x9cpushxe2x80x9d fans at one end draw air from the surrounding environment and exhaust the air into the enclosure while the xe2x80x9cpullxe2x80x9d fans at the other end of the system draw air out of the enclosure and exhaust the air into the surrounding environment. If sufficient numbers of fans are used, the airflow across the components will be high enough to prevent overheating even following the failure of a fan. This approach is generally successful, but does not work well in a system design where there is high airflow resistance over the components. For example, such a condition can occur when the distance between the printed circuit boards or component carriers is less than one inch.
The effect of this high resistance is that, when a fan fails, the airflow in that fan reverses, in turn, causing the air near a fan that has failed to recirculate around the fan. In particular, if a fan in the xe2x80x9cpushxe2x80x9d tray fails, some of the air pushed into the enclosure by the remaining push fans exits through the failed fan rather than flowing over the components. Similarly, if a fan in the xe2x80x9cpullxe2x80x9d tray fails, air is drawn through the fan into the enclosure by the remaining pull fans. The result is that the electronic components are deprived of sufficient airflow and a thermal runaway condition is created with resultant component failure.
A conventional solution to the aforementioned problem is to use impellers with backwardly-curved blades in place of fans in the xe2x80x9cpullxe2x80x9d tray. The impellers create an air flow that is strong enough to draw air through an opening occupied by any fan that has failed in the xe2x80x9cpushxe2x80x9d tray. However, when an impeller in the pull tray fails, the large hole it occupies makes it virtually impossible for the neighboring impellers to draw sufficient air through the component enclosure. Instead, the remaining impellers begin to draw air through the opening in the enclosure occupied by the failed impeller, resulting in airflow recirculation, this time in the pull tray, and again causing overheating.
Therefore, there is a need for a cooling system that can provide adequate airflow across component enclosures, including enclosures with high airflow resistance and that can tolerate the failure of a fan, or impeller, without overheating.
In accordance with one embodiment of the invention, impellers are used in the pull tray and are isolated from each other by baffles, so that each impeller its own exhaust air flow path. The exhaust path is provided with a predetermined airflow resistance. The exhaust airflow resistance is sufficiently low that the associated impeller can exhaust enough air to provide sufficient airflow over the components during normal operation. In particular, the airflow is sufficient so that if a fan in the push tray fails, the pressure drop created by the impellers in the pull tray is sufficient to draw air through the space occupied by the failed fan.
However, the exhaust airflow resistance for each impeller in the pull tray is sufficiently high that, if an impeller fails, the resistance limits the amount of air that the remaining impellers can pull in through the failed impeller opening, thereby enabling the airflow delivered by the fans in the push tray to continue to be distributed evenly across the components.
In one embodiment, the exhaust airflow resistance is created by using impellers with relatively small areas in the pull tray.
In another embodiment, the exhaust airflow resistance is created by placing a screen across the airflow exit to increase airflow resistance.
In still another embodiment, the exhaust airflow resistance is created by using a combination of impellers with relatively small areas and a screen across the airflow exit in order to increase airflow resistance.
The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which:
FIG. 1 is an exploded perspective diagram of a set of component carriers, a push tray and a pull tray, such as would be mounted inside an enclosure, but with the enclosure removed.
FIG. 2 is an enlarged perspective diagram of the inventive pull tray.